Well, with the icy conditions left over by the big storm, I did my first run on the treadmill in years - I walk and power hike inclines on the treadmill a lot, but I've managed to avoid running on it. I very much prefer to run outside in nearly any conditions.
The treadmill still feels so much harder to me than outside, even at the same alleged pace. It's not totally subjective - my HR was definitely quite a bit higher, increasing throughout the run, although it's warmer in my basement than conditions outside for my last few runs so that would contribute to the HR.
For kicks (it's a snow day again!), I measured the speed of my treadmill today by making a mark on the belt and counting how many cycles in a certain amount of time. I have a Spirit XT485 that is around 10 years old.
For a slow walking pace of 3.0 mph, repeated trials averaged a speed of 3.04 mph, or about 1.5% too fast.
For a slow running pace of 5.0 mph, repeated trials averaged a speed of 5.09 mph, or about 1.5% too fast, also.
I did the treadmill's factory calibration routine, and then the measurements went to 3.03 mph and 5.07 mph, respectively, which was a bit more accurate but not all the way there.
The treadmill manual gives instructions for taking it apart a bit and adjusting the "speed senors", but I'm not about to mess with that.
So I was right that my treadmill is too fast, but I don't think it's a big enough effect for me to actually be feeling it! I just really hate being on the treadmill.